Ch. 17 - The Endocrine System

17.1 Overview

The body has four ways that cells communicate with each other

1.        Gap Junctions

-              Join smooth muscle, cardiac muscle, epithelial, and other cells to each other. Ability for cells to pass nutrients, electrolytes, and signaling molecules directly from the cytoplasm of one cell to the cytoplasm if the next through pores in their plasma membrane

2.        Neurotransmitters

-              Released by neurons, diffuse across a narrow synaptic cleft, and bind to receptors on the surface of the next cell

3.        Paracrines

-              Secreted by one cell, diffuse to nearby cells in the same tissue, and stimulate their physiology. Some authorities call them local hormones

4.        Hormones

-              Chemical messengers that are transported by the bloodstream and stimulate physiological responses in cells of another, often distant tissue or organ.

Glands

-              Skull

o   (1) Pineal Gland, (1) Hypothalamus, (2) Pituitary gland

-              Neck

o   (1) Thyroid gland & (1) thymus

-              Abdomen

o   (1) Pancreas & (1) Adrenal Gland

-              Gonads

o   (1) Ovary & (1) Testis

Endocrine V. Exocrine Glands

-              Exocrine glands have ducts that lead to the epithelial surface.

-              Endocrine glands are ductless and release their product via bloodstream.

-              Endocrine glands have a high density of blood capillaries which are especially permeable, these are called fenestrated capillaries

-              Some organs are both endocrine and exocrine

Nervous System V. Endocrine systems

-              Differences

o   An efferent nerve innervates only on organ and a limited number of cells w/in an organ, so effects are precise & targeted.

o   Hormones will travel and circulate throughout the body – have more widespread effect.

o   The nervous system will typically start and stop in milliseconds. While in the endocrine system, it takes hormones to several seconds to days to act. Nervous system effects can stop immediately while endocrine effects can last for a long time.

-              Similarities

o   Both communicate chemically, and several chemicals function as both neurotransmitters and hormones: norepinephrine, dopamine, and antidiuretic

o   In some hormones and neurotransmitters produce identical effects on the same organ

§  Norepinephrine and glucagon both stimulate the liver to break down glycogen and release glucose

Target Cell Concept

-              Hormones cannot just go to a specific organ, therefore target organs or targets cells respond to it. These are cells that react to specific signals and use an enzyme to activate the hormone

17.2 The Hypothalamus & Pituitary Gland

 Anatomy

• The Hypothalamus

  • regulates primitive functions of the body such as water balance, thermoregulation, sex drive, & childbirth

  • Its functions are carried out by the pituitary gland

• The Pituitary Gland

  • suspended from the floor of the hypothalamus by a stalk (infundibulum) and housed in a depression of the sphenoid bone - the sella tunica.

  • Typically measures 12mm from left to right & 8mm from anterior to posterior

    • it grows about 50% larger in pregnancy

  • Composed of two structures

    • the anterior & posterior pituitary - w/ independent origins & separate functions

• The anterior pituitary (adenohypophysis)

  • arises from a pouch that grows upward from the embryonic pharynx

  • has no nervous connection to the hypothalamus but is linked to it by complex blood vessels called hypophyseal portal system

    • this is a network of primary capillaries in the hypothalamus, a group of small veins called portal venules that travel down the stalk, & a complex of secondary capillaries in the anterior pituitary

  • The hypothalamus controls the anterior pituitary by secreting hormones that enter the primary capillaries, travel down the venules, and diffuse out of the secondary capillaries into pituitary tissue.

• The posterior pituitary

  • arises as a bud growing downward from the brain

  • primarily composed of nervous tissue - not a true gland

  • the stalk that connects the pituitary to the hypothalamus is part of the posterior pituitary; the bulging part of the gland below the stalk is called the posterior lobe

  • hormones of the posterior pituitary are made by certain neuroendocrine cells in the hypothalamus

    • their axons pass down the stalk as a bundle called hypothalamo-hypophyseal tract & end in the posterior lobe

Hypothalamic Hormones (now the abbreviation & the name)

• Hormones produced in the hypothalamus

• 8 hormones - six to regulate the anterior pituitary & two stored in the posterior pituitary & released on demand

• The first six stimulate the pituitary to release hormones of its own are called releasing hormones; those that suppress pituitary secretion are called inhibiting hormones

• Hypothalamic releasing & inhibiting hormones that regulate the anterior pituitary

  • Thyrotropin-releasing hormones (TRH)

    • Promotes secretion of thyroid-stimulating hormones (TSH) & prolactin (PRL)

  • Corticotropin-releasing hormone (CRH)

    • Promotes secretion of adrenocroticotropic hormone (ACTH)

  • Gonadotropin releasing hormone

    • promotes secretion of follicle-stimulating hormone (FSH) & luteinizing hormone (LH)

  • Growth hormones-releasing hormone (GHRH)

    • promotes secretion of growth hormone (GH)

  • Prolactin-inhibiting hormone (PIH)

    • inhibits secretion of prolactin (PRL)

  • Somatostatin

    • inhibits secretion of growth hormone (GH) & thyroid -stimulating hormone (TSH)

• The other two hypothalamic hormones are oxytocin (OT) and antidiuretic hormone (ADH)

  • these two are stored & released by the posterior pituitary

  • OT mainly comes from neurons & left paraventricular nuclei of the hypothalamus

  • ADH mainly comes from the supraoptic nuclei, named fro their location about the optic chiasm

Anterior Pituitary Hormones

• The first two are collectively called gonadtropins bc they target ovaries & testes

• Anterior Pituitary Hormones

  • Follicle-Stimulating Hormone (FSH)

    • Target Organ/Tissue: Ovaries, testes

    • Female Effect: growth of ovarian follicles & secretion of estrogen

    • Male effect: sperm production\

    • Secreted by gonadtropic cells

  • Luteinizing Hormone (LH)

    • Target Organ/Tissue: Ovaries, testes

    • Female: ovulation, maintenance of corpus luteum

    • Male: Testosterone secretion

    • Secreted by gonadtropic cells

  • Thyroid-stimulating hormone (TSH)

    • Target Organ/Tissue: Thyroid gland

    • Effect: growth of thyroid, secretion of thyroid hormone

    • secreted by thyrotropic cells

  • Adrenocorticotropic hormone (ACTH)

    • Target: Adrenal cortex

    • effect; growth of adrenal cortex, secretion of glucocorticoid

    • secreted by corticotropic cells

  • Prolactin (PRL)

    • target: mammary glands

    • effect; milk synthesis

    • secreted by prolactin cells

  • Growth hormone (GH)

    • Target: liver, bone, cartilage, muscle, fat

    • effect: widespread tissue growth, especially in the stated tissues

    • secreted by somatropic cells

• Posterior Pituitary Hormones

  • Antidiuretic hormone (ADH)

    • Target: kidneys

    • Effect: water retention

  • Oxytocin (OT)

    • Target: uterus, mammary glands

    • Effect: labor contractions, milk release; possibly involved in ejaculation, sperm transport, sexual affection, & mother-infant bonding

• Relationship btw Hypothalamus & Pituitary Gland

  • Example: the hypothalamus secretes TRH which induces the anterior pituitary to secrete thyroid to secrete TSH; TSH, in turn, stimulates the thyroid gland to release TH; & finally the thyroid hormone exerts its metabolic effects throughout the body

  • This relationship is called an axis -the hypothalamo-pituitary-thyroid axis

Posterior Pituitary Hormones

• Antidiuretic Hormone (ADH)

  • increases water retention by the kidneys, reduces urine volume, helps prevent dehydration

  • also functions as a neurotransmitter & usually called arginine vasopressin (AVP)

    • also refers to its ability to cause vasoconstriction

• Oxytocin (OT)

  • best known as a powerful stimulus to labor contractions

  • sexual satisfaction, lactation, sexual arousal, maternal/paternal bonding

Control of Pituitary Secretion

• pituitary hormones are not secreted at constant rates

• Hypothalamic & Cerebral Control

  • both lobes of the pituitary gland are strongly subject to control by the brain

  • hypothalamic control enables the brain to monitor conditions w/in and outside the body and to stimulate or inhibit the release of anterior lobe hormones in response

• The posterior pituitary is controlled by neuroendocrine reflexes- the release of hormones in response to nerve signals

• Feedback from Target Organs

• Most often takes form of negative feedback inhibition

  • the pituitary stimulated another endocrine gland to secrete its hormone and that hormone feeds back to the pituitary or hypothalamus and inhibits further secretion of the pituitary hormone

• EX: Hypothalamno-pituitary-thyroid axis

  1. the hypothalamus secretes thyrotropin-releasing hormone (TRH)

  2. TRH stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH)

  3. TSH stimulates the thyroid gland to secrete thyroid hormone (TH)

  4. TH stimulates the metabolism of most cells throughout the body

  5. TH also inhibits the release of TSH by the pituitary

  6. To a lesser extent, TH also inhibits the release of TRH by the hypothalamus

A Further Look at Growth Hormone

• GH has widespread effects on the body

• Causes the liver to secrete insulin like growth factors (IGF-I and IGF-II)

• GH is short lived - 6-20min

• Mechanisms of GH & IGF

  • protein synthesis

  • lipid metabolism

  • carbohydrate metabolism

  • electrolytes balance

• Another stimulus to GH is ghrelin, a hormone secreted by the stomach, especially when empty

17.3 Other Endocrine Glands

The Pineal Gland

• attached to the roof of the third ventricle of the brain

• The pineal gland can shrink - this is called involution

• can respond to information from the eyes about the relative hours of light & darkness in the environment

• Synthesizes melatonin

  • plays a role in the body’s circadian rhythms, promotes sleep, and has antioxidant and anticancer properties. It can regulate gonads & sets the annual breeding season.

The Thymus

• Plays a role in three systems: endocrine, lymphoid, and immune

• Located between the lungs, superior to the heart, & behind the sternal manubrium.

• Site of maturation for T-cells

• Secretes thymopoietin, thymosin, & thymulin

  • stimulates the development of other lymphoid organs and regulates the development of activity of T cells

The Thyroid Gland

• largest adult gland

• adjacent to the trachea immediately below the larynx

• shaped like a butterfly wrapped around the trachea, joined inferiorly by a narrow bridge of tissue, the isthmus

• composed of thyroid follicles

  • each filled with a protein rich colloid and lined by a simple cuboidal epithelium of follicular cells

  • these secrete TH

    • about 90% is thyroxine T4, the other 10% is T3

• releases hormones directly into the blood stream

• Each follicle is surrounded by a network of capillaries

• TH to increase the metabolic rate, raises oxygen consumption and has a calorgenic effect - increases heat production

• contains parafollicular cells or clear cells

  • they respond to rising levels of blood calcium by secreting calcitonin

    • calcitonin antagonizes parathyroid hormones and stimulates osteoblast activity, thus promoting calcium desposition and bone formation

Parathyroid Gland

• ovoid gland, usually four in number, partially embedded in the posterior surface of the thyroid

• They secrete PTH which regulates blood calcium lvl

• directly monitor blood composition and secrete PTH when calcium level dips too low

  • raises calcium by stimulating calcium reabsorption from the bones and reducing calcium loss in urine

  • people can die if the parathyroid is removed

Adrenal Glands

• sit like a cap on the superior pole of each kidney

• Surrounding the adrenal medulla is the adrenal cortex

• the inner core is the adrenal medulla,

• Adrenal Medulla

  • Neuroendocrine cells

  • release their products into the bloodstream

  • Chromaffin cells release a mixture of catecholamines

    • epinepherine, norepinephrine, and dopamine

    • increase alertness and prepare the body in several ways for physical activity

  • liver boosts glucose lvls by glycogenolysis (hydrolysis of glycogen to glucose) and gluconeogenesis

  • Epinephrine has glucose-sparing effect; inhibits secretion of insulin so muscles and other insulin dependent organs absorb and consume less glucose

• Adrenal Cortex

  • produced more than 25 steroid hormones collectively called corticosteriods or corticoids

  • The five most important corticosteroids fall into three categories:

    • mineralcorticoids: regulate the body’s electrolyte balance

    • glucocorticoids: regulate metabolism of glucose

    • sex steroids: various developmental & reporductuve functions

  • Three layers of tissue make up the adrenal cortex:

    • Zona Glomerulosa: Thin layer; Source of mineralocorticoids

    • Zona Fasiculata: thick middle layer; secretes glucoorticoids

    • Zona Reticularis: narrow, innermost layer; secretes glucocorticoids and androgens

  • Aldosterone

    • mineralcorticoid

    • stimulated by falling blood pressure

    • stimulates the kidneys to retain sodium. Water is retained with it by osmosis, so aldosterone helps to maintain blood volume & pressure

  • Cortisol

    • most potent glucocorticoid

    • secreted in response to ACTH from the pituitary

    • stimulate fa & protein catabolism, glucogenesis, & release of fatty acids and glucose into the blood

  • Androgens

    • primary adrenal sex steroids

    • ACTH regulate both adrenal androgen & cortisol secretion

    • major androgen dehydroepiandrosterone (DHEA)

    • Estradiol

      • main adrenal estrogen

Pancreatic Islets

• Pancreas located btw the lower stomach and posterior body wall. It is a retroperitoneal gland.

• functions. as both endocrine and exocrine glands

• Glucagon

  • secreted by gluacagon cells or Alpha cells

  • from the pancreas

  • two primary actions

    • glycogenolysis, the breakdown of glycogen into glucose

    • Gluconeogenesis, the synthesis of glucose form fats and proteisn

  • raises blood glucose lvls

• Insulin

  • secreted by Beta cells or insulin cells

  • secreted during and immediately following a meal when blood nutrient levels are rising; even food aroma can stimulate insulin release

  • principle target are the liver, skeletal muscles, & adipose tissue

  • in times of plentiful food, insulin stimulates cells to absorb glucose, fatty acids, & amino acids and to store or metabolize them; therefore it lowers the level of blood glucose and other nutrients

  • promotes synthesis of glycogen, fat, and protein

• Amylin

  • secreted simultaneously with insulin

  • helps to reduce spikes. in blood glucose by slowing the emptying of the stomach

• Somatostatin

  • secrete somatostatin in response to glucose and amino acids

  • acts as a paracrine regulator of glucagon and insulin

• Pancreatic polypeptide

  • released 4 to 5 hours after. ameal

  • acts on receptors in the brain

  • inhibits stimulation of the pancreas by the vagus nerve, reducing secretion of pancreatic enzymes that are no longer needed

The Gonads

• functions as both endocrine and exocrine glands

  • exocrine products: eggs & sperm

  • Endocrine products: gonadal hormones

• Ovaries secrete chiefly estradiol, progesterone, & inhibin

• Testis’ endocrine secretions are testosterone

The Skin

• Keratinocytes in the epidermis convert a cholesterol-like steroid into cholecalciferol using energy for solar UV radiation

• The liver and kidneys further convert cholecalciferol to a calccium regulating hormone, calcitriol

The Liver

• It converts cholecalciferol from the skin into calcidiol the next step in calcitriol synthesis

• It secretes a protein angiotensinogen which the kidneys, lungs, and other organs concert to a hormone called angiotensin II part of the renin-angiotensin-aldosterone system

• It secretes 15% of the body’s erythropoietin a hormone that stimulates RBC production in the bone marrow

• Secretes insulin-like growth factor that mediates the action of growth hormone

• secretes hepcidin the principal hormone of iron metabolism

The Kidneys

• They convert the calcidiol into calcitriol (Vitamin D3) which promotes intestinal absorption and lessens urinary loss of calcium

• They secrete an enzyme called renin which converts angiotensin I into angiotensin II a small peptide which constricts blood vessels throughout the body

• They secrete 85% of the body’s eyrhtropoietin. In renal failure the EPO output falls causing the anemia of renal failure

The Heart

• Rising blood pressure and excess fluid retention stretches the heart and stimulates cardiac muscle in the atria to secrete two similar natriuretic peptides

• These peptides increase sodium excretion and urine output and oppose the action of angiotensin

• Together, these effects lower blood volume and blood pressure

Skeletal Muscles

• The muscles secrete a family of hormones called myokines

• They act on the liver and adipose tissue to mobilize high energy fuels by stimulating fat breakdown (lipolysis) and glucose synthesis (gluconeogenesis)

The Stomach and Intestines

• They contain the largest endocrine network in the body

• There are more than 15 kinds of enteroendocrine cells, constituting 1% of all epithelial cells of the GI tract

• They secrete enteric hormones that effect feeling, digestion, GI motility secretion, and maintenance of the mucosa

• Gut-brain peptides effect both the gut and brain

• Peptide YY signals satiety terminating eating

• Cholecytokinin responds to fat stimulating gallbladder to release bile

• Ghrelin is secreted when the stomach is empty, stimulates appetite and release of GHRH

• Gastrin upon arrival of food in the stomach stimulates other cells in the stomach to release HCl

Adipose Tissue

• secrete at lease three hormones that regulate carbohydrate and fat metabolism

• A low level of leptin increases appetite whereas a high level of leptin tends to blunt the appetite

• Leptin also serves as a signal for the one of puberty which is delays in people with an abnormally low body fat

Osseous Tissue

• Osteoblasts secrete the hormones osteoclacin and lipocalin

• both stimulate pancreatic beta cells to promote insulin secretion, playing a role in glucose metabolism and regulation of blood glucose levels

The Placenta

• The organ preforms many functions in pregnancy including fetal nutrition, oxygenation, and waste removal

• It secretes estrogen, progesterone, and other hormones that regulate pregnancy and stimulate development of the fetus and the mother’s mammary glands

BE SPECIFIC WHEN STATING LOCATION

17.4 Hormones & Their Actions